EFFLUENT MIXING ZONE SUDY OF A LARGE RIVER

The City of Lawrence, Kansas wastewater treatment plant (WWTP) discharges to the Kansas River. Farmland Industries discharges process water of high ammonia concentration to the river approximately 1.3 miles downstream of the Lawrence WWTP. The Kansas Department of Health and Environment (KDHE) recently completed a Total Maximum Daily Load (TMDL) study for ammonia and developed wasteload allocations (WLA) for the two discharges, assuming a combined mixing zone.KDHE's assumption of a combined mixing zone was questioned because the City's effluent may have mixed with a significant amount of the flow in the river before reaching the Farmland effluent. A separate WLA calculation for the WWTP could result in less stringent ammonia limits than previously calculated by KDHE using the combined mixing zone.The City contracted with Black & Veatch to conduct a site-specific mixing zone study to collect data to determine the extent of effluent mixing in the river between the WWTP and Farmland's discharge point. The study involved injection of a fluorescent dye into the WWTP effluent and measurement of the dye concentrations in the river using a fluorometer. The dye study results were used to calibrate the CORMIX model, which predicted the amount of dilution and mixing at regulatory low flow in the river.